Gapped quantum spin liquid in a triangular-lattice Ising-type antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>PrMgAl</mml:mi><mml:mn>11</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</

In the search of quantum spin liquid (QSL), spin-1/2 triangular-lattice Heisenberg antiferromagnets have always been viewed as fertile soils. Despite the true magnetically ordered ground state, anisotropy has been considered to play a significant role in stabilizing a QSL state. However, the nature and ground state of the most anisotropic case, the triangular-lattice Ising antiferromagnet (TLIAF), remain elusive and controversial. Here, we report specific-heat and thermal-conductivity measurements on a newly discovered Ising-type QSL candidate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>PrMgAl</a:mi><a:mn>11</a:mn></a:msub><a:msub><a:mi mathvariant="normal">O</a:mi><a:mn>19</a:mn></a:msub></a:mrow></a:math>. At zero field, the magnetic specific heat shows a quadratic temperature dependence. On the contrary, no direct positive magnetic contribution to thermal conductivity is detected, ruling out the presence of mobile gapless fermionic excitations. Further analysis of phonon thermal conductivity reveals that the phonons are strongly scattered by thermally activated magnetic excitations out of a gap, which exhibits a linear dependence with magnetic field. These results demonstrate that the spin-1/2 TLIAF <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi>PrMgAl</c:mi><c:mn>11</c:mn></c:msub><c:msub><c:mi mathvariant="normal">O</c:mi><c:mn>19</c:mn></c:msub></c:mrow></c:math> has a gapped <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>Z</e:mi><e:mn>2</e:mn></e:msub></e:math> QSL ground state. Published by the American Physical Society 2024